The present invention relates generally to plant molecular biology and genetics and to nucleic acids and methods for conferring resistance to bacterial disease in plants. The present invention also relates to promoters and promoter sequences useful for controlling expression in transgenic plants.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the Bibliography.
Gram-negative phypathogenic bacteria employ a type III secretion system (TTSS) to translocate effector proteins into plant cells where they modulate host cell functions for the benefit of the invasion process (He et al., 2004). Pathovars of Xanthomonas and Ralstonia solanacearum harbor members of the large AvrBs3 effector family (Schornack et al., 2006; Heuer et al., 2007). AvrBs3-like effectors, also referred as transcription activator-like (TAL) type III effectors (Yang et al., 2006), are remarkably similar. Each possesses a central near-perfect repeats of a 34-amino acid sequence that vary in repeat number, imperfect heptad leucine zipper (LZ) repeats, three highly conserved C-terminal nuclear localization signals (NLS), and C-terminal acidic transcription activator-like domains (AAD). Several studies indicated that the TAL effectors from Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas campestris pv. vesicatoria (Xcv) specifically activate the cognate host genes for promoting disease susceptibility (Yang et al., 2006; Sugio et al., 2007; Kay et al., 2007) or triggering disease resistance (Gu et al., 2005; Romer et al., 2007). The code of DNA binding specificity of TAL effectors was broken recently based on the detailed characterization of the conserved AvrBs3 binding sites in the promoters of the cognate Bs3 and upa genes from pepper (Capsicum annuum) as well as several other TAL effector binding sites (Kay et al., 2009; Romer et al., 2009a; Boch et al., 2009). According to the proposed model, each TAL effector repeat including the most C-terminal half repeat, which shows hypervariable amino acids at position 12 and 13, specifically recognizes a nucleotide in the DNA binding site of host gene promoter with a conserved T at the 5′ end (Boch et al., 2009).
Bacterial blight of rice, caused by Xoo, is one of the most destructive bacterial diseases of rice, prevalently in irrigated and rainfed lowland rice growing areas throughout Asia (Mew, 1987). The utilization of host genetic resistance is the most economic and effective way to control this disease. Over 30 resistance (R) genes or loci with race-specific resistance to Xoo were identified in cultivated and wild rice (Nino-Liu et al., 2006). Six of them have been cloned and their gene products show great diversity (Chu et al. 2006; Gu et al. 2005; Iyer and McCouch, 2004; Song et al., 1995; Sun et al., 2004; Yoshimura et al., 1998). R gene Xa27 (Gu et al., 2005) and disease-susceptibility gene Os8N3 or the susceptible allele of the recessive R gene xa13 (Yang et al., 2006) were found to be specifically induced by TAL effectors AvrXa27 and PthXo1, respectively. The binding sites of the two TAL effectors in the promoters of their cognate host genes were identified recently (Romer et al., 2009b; Boch et al., 2009). A recent genetic study indicated that the general transcription factor OsTFIIAγ5 is required for AvrXa27 to fully activate Xa27 transcription in rice and Xa27-mediated disease resistance to bacterial blight (Gu et al. 2009).
The bacterial blight R gene Xa10 was originally identified from rice cultivar Cas 209 (Mew et al., 1982; Yoshimura et al., 1983) and was later introgressed into susceptible rice variety IR24 (Ogawa et al., 1988). The cognate avrXa10 gene from Xoo strain PXO86 encodes a member of TAL type-III effectors (Hopkins et al., 1992). The Xa10 locus was initially mapped to the long arm of chromosome 11 (11L) in the region between proximal RAPD marker O072000 (5.3 cM) and distal RFLP marker CDO365 (16.2 cM) (Yoshimura et al., 1995). It was recently mapped at genetic distance of 0.28 cM between proximal marker M491 and distal marker M419 and co-segregated with markers S723 and M604 (Gu et al., 2008).
Thus, it is desired to develop nucleic acids and methods for conferring resistance to bacterial disease in rice and other plants. It is also desired to develop isolated promoters or promoter sequences that can be used in genetic engineering of rice and other plant species.